Kohler K91 Service Manual page 108

Leaka g e : A poor grind on face or seat of valve will
allow leakage resulting in a b urned valve on one
side only.
Coking is normal on intake valves and is
Cokin g :
not harmfu l . If the seat is good . the valve could
be reused after cleaning .
Ove r heatin g : An exhaust valve subject to over­
heating will have a dark discoloration in the area
above the valve guide . Worn guides and faulty
valve springs may cause this condition . Also check
for clogged air intake . blocked fins. and lean fuel
mixture when this condition is noted .
Excessive build up of deposits in the
Carbon Cut:
combustion chamber may result in valve damage
because deposits can become hard enough to cut
the valve . Cleaning of the cylinder head at proper
intervals could prevent such damage .
Ste m Corrosion: Moisture from the fuel or from
condensation are the most common causes of
valve stem corrosio n . Condensation occurs from
improper preservation during storage and when
engine is repeatedly stopped before it has a
chance to reach normal operating temperatures.
Replace corroded valves .
G u m : Gum deposits usually result from using stale
gasoline . This condition is often noted in applica­
tions where fuel is not d rained out of the tan k dur­
ing the off season . Gum is a prevalent cause of
valve sticking . The cure is to ream the valve
guides and clean or replace the valves , depending
on their condition .
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